The present invention relates generally to an apparatus for thermal coagulation and more particularly to a system for applying heat to the endometrium of a human uterus of the type comprising a heated balloon catheter having a rotary impeller for improved circulation of fluid within a distensible bladder.
The following terms as used herein have the meaning given below:
"Menorrhagia" means a condition of excessive menstrual bleeding in women.
"Thermal coagulation" means the application of heat to tissue in an amount sufficient to destroy the tissue.
"Necrosis" means the death of cells in tissue.
"Endometrium" is that portion of the inner lining of the uterus to which an embryo normally attaches and is responsible for the menstrual cycles.
Apparatus and methods utilizing heated balloons or similar distensible bladders have been used to treat menorrhagia in women. Patients and physicians may prefer treatment of menorrhagia with a heated balloon, because such a minimally invasive procedure effectively curtails the excessive uterine bleeding associated with menorrhagia without requiring surgical removal of the uterus. Such balloon therapy involves inserting and inflating a balloon with a fluid into the uterus. After balloon inflation, the fluid is heated to a temperature for a period of time that coagulates, ablates, necroses, or destroys the endometrium (mucous membrane) and perhaps a portion of the myometrium (muscular layer). A successful endometrial ablation procedure requires controlling the temperature of the balloon. If the heating of the endometrial lining continues longer than necessary for thermal coagulation of the endometrium, then the myometrium could be irreparably damaged.
Temperature fluctuations and gradients along the surface of the balloon adversely affects an operator's control over endometrial thermal coagulation. Temperature fluctuations and gradients are, in part, caused by convection currents of the fluid within the balloon and the presence of an insulating, static boundary layer of fluid along the inner wall of the balloon. While cooler fluid moves toward the bottom of the balloon, the warmer, less dense fluid rises. When the fluid within the balloon is subject to such convection during heating, considerable temperature fluctuations along the surface of the balloon may result, causing less than optimal results. Mechanical circulation or agitation of fluid within the balloon has been known to improve the temperature consistency over the surface of the balloon.
Some balloon catheters circulate fluid by means of separate inlet and outlet passages that connect the balloon with an external heating element. Heat is circulated from the external heating element through the inlet passage into the balloon. Then, the fluid from the balloon is returned to the external heating element through the outlet passage. Such a catheter design requires the hot fluid to pass through the vagina and the opening of the cervix, which may cause physical discomfort or possible tissue damage as heat is conducted through the catheter walls. Since the hot fluid must travel a significant distance between the external heating element and the balloon surface being heated, control over temperature of the balloon surface is difficult.
Other known heated balloon catheters circulate fluid via a pair of one way valves mounted within a housing located at the end of a fluid delivery tube. The housing is surrounded by an inflatable member, such as a balloon. The first valve permits fluid flow from the housing into the balloon. The second valve permits flow from the balloon into the housing. The valves respond to alternating pressure differentials between the balloon and the housing created by an external bellows or piston which causes pulses of fluid to move up and down the fluid delivery tube. Such a configuration requires circulating hot fluid from the balloon into the fluid delivery tube, creating a risk of causing discomfort to the patient or vaginal tissue damage.
Another balloon catheter design known in the art places a propeller or pump wheel within a lumen of a tubular housing contained within the balloon. Such a configuration creates axial fluid motion or motion substantially parallel to the axis of rotation. However, because the propeller is contained within the housing, any axial fluid flow results in mostly linear flow through the tubular housing and a generally linear current within the balloon. Thus, the heating of the balloon surface may not be uniform and the fluid may not properly circulate around the cornua of the uterus where the endometrium is usually the thickest. Another problem with this approach is the lack of sufficient cross section of the lumen to prevent a sufficiently low resistance to passing the fluid through the housing and into the balloon. Thus, vigorous circulation may not be possible so as to prevent a boundary layer of fluid from forming along the inner surface of the balloon.
Other balloon catheter configurations which have limited effectiveness or practicality are known. One such design places a longitudinally vibrating member at the end of a heating element within a balloon. Another design places a flat shape memory alloy at the end of the heating element, such that the shape memory alloy responds to electrical impulses to move the alloy in a lateral fanning motion, thereby somewhat circulating the fluid within the balloon. Each of these designs may work with varying degrees of effectiveness, but have yet to provide a practical configuration and cost effective solution for providing uniform heating of the balloon surface. Thus, heretofore, there was a need for a circulation system that causes vigorous agitation of fluid within the distensible bladders of a balloon catheter in a safe and effective manner.